The present invention relates to a jacking system for jacking a structure, for instance offshore platforms or vessels, out of the water, and a method for jacking using this system.
Such a jacking system is for instance disclosed in U.S. Pat. No. 3,804,369. This jacking system has pairs of hydraulic cylinders which are mutually coupled using wire cables.
Another such jacking system is disclosed in U.S. Pat. No. 4,007,914. This system has three mutually fixed frame parts. One end of a hydraulic cylinder is fixed to one frame part, one end of another hydraulic cylinder is fixed to a next frame. The other ends of the hydraulic cylinders are coupled.
Yet another such jacking system is disclosed in U.S. Pat. No. 4,411,408. This jacking system has a leg which has several “chords” each having two hydraulic cylinders at opposite sides of a chord. Both ends of the hydraulic cylinders have a retractable pin which can be extended into passages in a chord. Both cylinders displace in a synchronised fashion.
GB-2.004.246 discloses a jacking system with square legs with holes and a frame which is attached to a platform which may have two yokes at two opposite sides of a leg and which have pins which are insertable in the holes of the legs. The frame has locking pins at the same sides of the actuators. These limit the stroke of the yokes.
GB-2.004.247 discloses a jacking system with square legs with holes and a frame which is attached to a platform which has two yokes at opposite sides of a leg and which have pins insertable in these holes. The yokes can pass one another and thus together provides a fast leg handling. Handling of high loads may be a problem, as well is simultaneously handling several legs on one platform.
FR-A-2.235.602 discloses a platform provided with legs which can be lowered by filling the legs with ballast, and raised by removing ballast. When the legs rest on the bottom of the sea, the platform can be raised via a jacking system. The legs have reservoirs for storing oil, for instance, and are made of concrete. Each leg has four actuators. The actuators are positioned within the platform, thus limiting their stroke to the thickness of a platform. All four actuators have the same stroke, and can all only cover one pitch distance. Furthermore, it has locking pins at fixed positions.
U.S. Pat. No. 2,932,486 discloses a jacking system for jacking a platform. It has legs provided with rails with holed. Furthermore, it has double acting hydraulic cylinders with locking pins at one end of the cylinders and locking pins at the opposite end of the cylinders on a frame attached to the platform. It uses two or three actuators for each leg. Furthermore, the actuators have identical maximum working strokes.
U.S. Pat. No. 3,056,585 discloses a pontoon with legs which can be raised and lowered. Each leg is square and has at each corner a first hydraulic cylinder which can extend in a first direction and a second hydraulic cylinder which can extend in the opposite direction. The ends of two adjacent first cylinders are coupled via a beam, and the second cylinders that in line are also coupled via a beam. Those beams engage in recesses of rails provided at the four corners of a leg. Thus, there are in fact four actuators, with two actuator at the same side of a leg, and in line. The two actuators move in opposite directions. The entire actuator system thus extend along a considerable length of a leg.
All these systems thus have several disadvantages regarding jacking speed, holding efficiency, compactness or complexity. Some systems need legs which are longer than strictly needed. Others need relatively high frames or lack stability. There is, therefore, room for improvement.